Non-pigmented white coating



Dec. 15, 1964 J ALSUP 3,161,519

NON-PIGMENTED WHITE COATING Filed June 14, 1961 MICHAEL J. ALSUP INVENTOR.

ATTORNEY Q /IIGENT United States Patent Ofiice 3,161,519 Patented Dec. 15, 1964 3,161,519 NON-PIGMENTED WHITE CUATING Michael J. Alsup, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New .lersey Filed June 14, 1961, Ser. No. 116,952 8 (Ilaims. (Cl. 96-85) This invention concerns White coatings. More particularly, this invention concerns white non-pigmented coatings for use on photographic paper as a substitute for the normal pigmented layer.

Photographic print paper generally is coated with a baryta layer consisting of barium sulfate dispersed in gelatin. This layer has the purpose of providing a smooth, white, reflecting surface for the subsequent emulsion coat ing. It also has the function of covering or hiding the paper fibers due to its inherent opaqueness to light.

Certain properties of the baryta layer on photographic paper, however, are undesirable and must be compensated for, on some grades, by subsequent sizing operations or by built-in compensations in the photographic emulsion itself. For instance, the barium sulfate if carried in a binder other than gelatin may in some instances sensitize the photographic emulsion and result in very small black spots which are unacceptable in the final photographic product. Moreover, the amount of barium sulfate which is required for pigmenting purposes may result in a very stiff inflexible coating so that when the paper is folded or creased, the coating is severely broken; in other instances, the coating cannot resist abrasion or handling to any extent.

Normal baryta coatings in some instances have been rougher than desirable. In other instances, the processing steps involving immersing the photographic print in liquid developers and processing solutions has resulted in a rougher surface. Furthermore, the requirements that the baryta be suspensed in a gelatin binder rather than certain other types of binders which are more resistant to moisture penetration or moisture damage, has been extremely limiting to the use of baryta as a pigmenting material.

I have found a method of providing a White coating on photographic paper involving the use of a polymeric composition along with colloidal silica which provides a white background having good adhesion to photographic emulsion and which avoids the difiiculties previously experienced in using baryta coatings.

One object of this invention is to provide a non-pigmented white coating for photographic paper. A further object is to provide a process for coating photographic paper with a white coating without using baryta. Another object is to provide a photographic paper with a surface smoother than that normally used. An additional object is to provide a formaldehyde-free white coat for photographic paper.

The above objects are obtained by coating photographic paper base stock with a coating composition containing polyvinyl alcohol, a colloidal dispersion of silica, and a small amount of an acid containing, cross linked, acrylic emulsion copolymer. Precipitation which is not reversible occurs in this coating composition when the coating is dried down.

Certin other materials may be advantageously added to the coating composition such as plasticizers, spreading agents, defoamers, additives, and the like.

The particle size of the colloidal silica may vary between certain diameters. However, in our preferred embodiment, the particle size within the range of -100 millimicrons is used.

The acrylic polymer can be either polyacrylic acid or a copolymer of at least one lower alkylacrylate in which the alkyl group has from 1 to 4 carbonatoms with at least 25% by weight in the copolymer of at least one acid selected from acrylic acid and methacrylic acid. In our preferred embodiment, we use a terpolymer of ethylacrylate, styrene, and acrylic acid having acrylic acid in the proportion of about 33 percent.

The binder referred to herein as polyvinyl alcohol is a partially hydrolyzed polyvinyl alcohol containing approximately 10-15 percent of unhydrolyzed polyvinyl acetate. Gelatin and a styrene-butadiene resin may also be used in the same ratios as the polyvinyl alcohol.

Plasticizers may be such coating plasticizers as glycerol, ethylene glycol, etc. These materials are used to make the coating more flexible, thus inhibiting cracking of the coating. The operating range of these materials is 0.45- 1.0 percent based on the total Weight of the coating composition.

Spreading agents such as the alkyl sulfates and alkyl sulfonates may be used as well as others which are resin derivatives and the like. These components help in spreading the coating solution, especially if the coating is to be applied on a hydrophobic surface such as polyethylene. The operating range is 0.07-1 percent based on the total weight of the coating solution depending on the hydrophobicity of the coating support.

Defoamers such as butyl alcohol, certain phosphorous derivatives, etc., may be used in an amount of 0.2-5 percent based on the total weight of the coating solution depending on the method of agitation used in the solution makeup.

Polyvinyl acetate may be used as a partial replacement for or in addition to, polyvinyl alcohol. It serves as another binder in the system. The operating range is 1.3-6.5 percent based on the dry weightof the coating.

The pigment-to-binder ratio of 5:1 to 20:1 may be used in which polyvinyl alcohol or the like is considered a binder and the pigment or solids content refers to the copolymer plus the colloidal silica. The composition of the solids content can be from .4-5 percent by weight copolyrner with the balance of the solids colloidal silica.

This coating composition may be used as a subbing for polyethylene coating using a low coverage of 2-4 grams.

A colloidal silica-gelatin mixture may also be used to coat the surface of polyethylene or similar materials. The colloidal silica-gelatin mixture may be applied from an aqueous dispersion of water soluble gelatin and col loidal silica by any conventional coating method and dried by hot air at F. to F. The silica-gelatin coated polyethylene surface is hydrophilic, and photographic emulsions can be coated over it with good adhesion to it. From 0.7 to 1.0% by weight gelatin is used in the coating composition, the coverage applied on the hydrophobic support may vary from 2 .to 7 grams per square meter of the colloidal silica used alone as an aqueous subbing or with the addition of 0.7-1.0% gelatin and small amounts of other additives. A polyamide wet strength agent may be used in a silica-gelatin coating to provide good adhesion of a photographic emulsion to polyethylene with a thinner silica-gelatin layer than if the polyamide were not present.

In our preferred embodiment the colloidal silicapolyacrylic acid coating is used with partially acetylated paper made as described in US. patent applications Serial No. 786,041, now abandoned, filed January 12, 1959, entitled Paper of Good Dimensional Stability, in the names of William H. Griggs et a1. and Serial No. 62,900, filed October 17, 1960, entitled Paper of Good Dimensional Stability, in the names of William H. Griggs et al.

The accompanying drawing shows in a greatly enlarged cross-sectional View a representative photographic element of the invention wherein layer 10 is a paper support carrying a layer 11 of polyethylene and a photographic emulsion 13 adhered thereto by means of a layer 12 of a mixture of colloidal silica and acrylic acid polymer.

The following examples are intended to illustrate our invention but not to limit it in any way:

Example 1 A coating mixture was made up as follows:

Percent by weight Colloidal silica 30% 44.60 Gelatin .70 Wetting agent .20 Water 54.50

This mixture was coated on untreated polyethylene coated paper and dried with hot air at 130 F. to 150 F. The samples were then coated with a photographic emulsion and good adhesion was obtained. On untreated polyethylene coated paper no adhesion was obtained.

Example 2 A coating mixture was made up as follows:

Percent by weight Colloidal siica 30% 40.00 Gelatin .60 Wetting agent .15 Water 59.25

This mixture was coated on untreated polyethylene coated paper and dried with hot air at 150 F. The coating coverage was 3.0 grams per square meter. A photographic emulsion was coated on the treated paper with good adhesion.

The same emulsion adhered poorly to polyethylene coated paper treated with either colloidal silica or colloidal silica-latex and would not adhere at all to an untreated polyethylene surface.

Example 3 A coating mixture was made up as follows:

Percent by weight Colloidal silica 30% 47.50 Gelatin .70 Wetting agent .20 Water 51.60

The mixture was coated on untreated polyethylene coated paper and dried with hot air at 130-140" F. The adhesion of a photographic emulsion to this coating was very good. The coverage of this silica-gelatin coating was 4 grams per square meter.

Example 4 A coating mixture was made up as follows:

Percent by Weight Ludox HS, 30% 46.00 Gelatin .69 Kymene, 1 10% .34 Wetting agent .20 Water 52.77

Kymene is a polyamide prepared by reacting adipic acid with diethylene triamine and subsequently treated with emchlorohydrin. The resulting product has the structure:

(5:0 CH a The mixture was coated on untreated polyethylene coated paper and dried with hot air at 130140 F. This mixture was coated to give a coverage of 2.0 grams per square meter. This paper was coated with a photographic emulsion with excellent adhesion results. Silica, silicalatex, and silica-gelatin coatings exhibited only poor adhesion with emulsions when coverage was 2.0 grams per square meter.

Example .5

An opaque craze-free coating was obtained on polyethylene coated paper from dope prepared as follows:

Parts by weight Colloidal silica (particle size 5-7 mp1) (30 percent solids) Acrylic acid copolymer (28%) 5 Alkyl sulfonate wetting agent (10%) 2 In the dried down condition 95.5 percent was colloidal size silica and 4.5 percent was the acrylic acid copolymer.

Various other ratios of colloidal silica to the acrylic acid resin were prepared and coated where the ratios of slica to polymer ranged from 21:1 to 210:1. The higher the concentration of the acrylic acid resin with respect to the silica, the more opaque the film was in the dried down condition.

Example 6 The following formulation was coated on raw stock and on partially acetylated paper at 15 and 30 grams per square meter coverages from a 15 percent solution. An extrusion hopper was used to apply the coat.

Parts by weight Colloidal silica (5-7 millimicrons particle size) (30%) 200.0 Acrylic resin (terpolymer of about 50% ethyl acrylate, 17% styrene, and 33% acrylic acid) (28%) 5.0 Polyvinyl alcohol (5%) 15.0 Butyl alcohol 10.8

Alkyl sulfonate (10%) Samples of this coating formulation on partially actylated paper stock were sensitized using a chlorobromo silver halide emulsion and processed. The coated paper has excellent surface characteristics including good adhesion to the emulsion.

Example 7 Examples 5 and 6 were repeated except that the soluble salt of the acrylic resin was prepared by diluting with water and neutralizing with sodium hydroxide prior to mixing with the colloidal silica.

Example 8 The opacity of this coating composition could be increased by the addition of a white pigment such as barium sulfate, titanium dioxide, etc. A representative formulation which was prepared and coated by both dispersion hopper and air knife methods with excellent results is as follows:

Samples of this formulation in a dried-down conditwo were sensitized using a silver chloride light sensitive emulsion. Excellent adhesion results were obtained.

Coverages ranged from to 57 grams per square meter. The glycerol added to this formulation improves the flexibility and the defoaming agent reducing the foam. Polyvinyl acetate resulted in good film strength and stock adhesion.

Example 9 Example 8 was repeated except that titanium dioxide Was used in place of barium sulfate with satisfactory results.

Example 10 Example 8 was repeated except that a 3:1 ratio of colloidal silica to barium sulfate was used.

Example-11 Examples 6 and 8 were repeated except that gelatin was used in place of polyvinyl alcohol. A small amount of ammonium hydroxide was added to the gel prior to mixing with the silica in order to provide the proper pH values.

The acrylic resins used in my process are water soluble at a pH of 5 and above. In the formulations quoted herein where various percentages are indicated of the solutions, water is the solvent or dispersing agent used therein or as otherwise designated. For instance the acrylic resin is prepared with a Water dispersion. The colloidal silica is dispersed in water, and the like.

It will be clear to those skilled in the art that various methods of coating may be used including knife, hopper, comb bar, and the like. Moreover, various coating emulsions may be applied depending on the purpose intended. As pointed out above, a thin subbing is used for sizing polyethylene and similar thin sizings may be used on polypropylene and other substrates requiring such a sub. However, in the event that a heavy coating is desired, adhesion can be considerably improved by treating with a known method such as electron bombardment, flame oxidizing, etc.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A polyethylene-coated photographic paper having thereon a coating composition comprising, based on the dry weight of solids on the composition 95-99.6% by weight of colloidal silica the particles of which are about 5-100 millimicrons in size and 0.4-5% by weight of a polymer selected from the class consisting of homopolyacrylic acid and a copolymer of at least one acid selected from the group consisting of acrylic acid and methacrylic acid with further ethylenic unsaturated monomer.

2. A polyethylene-coated photographic paper having thereon a coating composition comprising, based on the dry weight of solids on the composition, 95-99% by weight of colloidal silica the particles of which are about 5-100 millimicrons in size and 0.4-5% by weight of a copolymer of acrylic acid with further ethylenic unsatu rated monomer.

3. A polyethylene coated photographic paper having thereon a coating composition comprising, based on the dry weight of solids in the composition, 95-99.6% by weight of colloidal silica the particles of which are about 5-100 millimicrons in size and 0.45% by weight of homo polyacrylic acid.

4. A polyethylene coated photographic paper having thereon a coating composition comprising, based on the dry weight of solids in the composition, 95-99.6% by weight of colloidal silica the particles of which are about 5-100 millimicrons in size and 0.4-5% by weight of a copolymer of polymethacrylic acid with further ethylenic unsaturated monomer.

5. A polyethylene-coated photographic paper having thereon a coating composition comprising, based on the dry Weight of solids in the composition, 96-99.6% by Weight of colloidal silica the Particles of which are about 5-100 millimicrons in size and 0.4-5% by weight of a polymer selected from the class consisting of homo 'polyacrylic acid and a copolymer of at least one acid selected from the group consisting of acrylic acid and methacrylic acid with further ethylenic unsaturated monomer, and having thereover a silver halide emulsion.

6. A coating composition comprising 45-20% by Weight polyvinyl alcohol, -90% by weight colloidal silica and 0.4-570 by weight of a polymer selected from the class consisting of polyacrylic acid and a copolymer of at least one acid selected from the group consisting of acrylic acid and methacrylic acid with further ethylenic unsaturated monomer.

7. A coating composition comprising 45-20% by weight polyvinyl alcohol, 75-90% by Weight colloidal silica and 0.4-5 by weight of an acid containing crosslinked, acrylic emulsion copolymer.

8. A coating composition comprising 45-20% by weight polyvinyl alcohol, 75-90% by Weight colloidal silica and 0.4-5 by weight of polyacrylic acid.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A POLYETHYIENE-COATED PHOTOGRAPHIC PAPER HAVING THEREON A COATING COMPOSITION COMPRISING, BASED ON THE DRY WEIGHT OF SOLIDS ON THE COMPOSITION 95-99.6% BY WEIGHT OF COLLOIDAL SILICA THE PARTICLES OF WHICH ARE ABOUT 5-100 MILLIMICRONS IN SIZE AND 0.4-5% BY WEIGHT OF A POLYMER SELECTED FROM THE CLASS CONSISTING OF HOMOPOLYACRYLIC ACID AND A COPOLYMER OF AT LEAST ONE ACID SELECTED FROM THE GROUP CONSISTING OF ACRYLIC ACID AND METHACRYLIC ACID WITH FURTHER ETHYLENIC UNSATURATED MONOMER. 